Folliculitis Decalvans: When to Repeat Biopsy or Culture?

Living with the repeated flares of folliculitis decalvans (FD) can be frustrating. Because FD is a chronic condition, not every new flare requires a repeat scalp biopsy or bacterial culture ^[1][2]. However, repeat testing may be considered if your symptoms do not respond to prescribed treatments, if there is a disagreement between how your scalp looks and your previous pathology results, or if you develop new, unusual symptoms ^[3][4]. Your doctor will individualize the decision based on how your condition is behaving, so it is helpful to bring a medication list, dates of recent antibiotics, and photographs of your flares to your appointments.

When to Consider a Repeat Scalp Biopsy

A scalp biopsy is a procedure where a small piece of skin is removed and examined under a microscope (pathology) ^[5]. This procedure is typically done under local anesthetic, may require a small stitch, and can leave a tiny scar. While a single biopsy is often enough to diagnose classic FD, a repeat biopsy may be useful in the following situations:

• Persistent Activity Despite Treatment: If your condition continues to worsen despite taking treatments exactly as prescribed, a repeat biopsy can help confirm if another condition is mimicking FD ^[6][7].
• Possible FD-LPP Overlap: Sometimes, FD can occur at the same time as another scarring hair loss condition called lichen planopilaris (LPP) ^[8][7][9]. If your doctor notices new signs like scaly skin around the hair follicles, a repeat biopsy can help identify this overlap, which may require a different treatment approach ^[10][7].
• Disagreement Between Clinical Signs and Pathology: The inflammation in FD can be patchy, and a biopsy might sometimes miss the characteristic features (like a high number of white blood cells called neutrophils) ^[11][12][13]. If your scalp looks like FD but your first biopsy was inconclusive, a repeat biopsy may be necessary ^[3][4].
• New Red-Flag Features: If you develop a rapidly growing lump, a non-healing ulcer, persistent bleeding, or an unusually painful nodule, you should seek prompt in-person evaluation by a dermatologist. Squamous cell carcinoma (a type of skin cancer) has rarely been reported in long-standing areas of scarring alopecia, including FD ^[14][15][16]. If malignancy is suspected, the suspicious lesion itself must be assessed and sampled.

Where should the repeat biopsy be taken?

To get the most accurate results for diagnosing FD or LPP, a biopsy should target an active edge—an area where the inflammation or hair loss is currently spreading ^[17][11]. Biopsying the center of an old, scarred area (end-stage lesion) often provides little information because the inflammation has already burned out ^[17][5]. Your doctor may use a specialized magnifying tool called a dermatoscope (trichoscopy) to closely examine your scalp and select the best active site ^[11][5]. In some cases, clinicians may take more than one punch biopsy or request specific sectioning methods from the lab to get a full view of the hair follicles ^[12][11].

When to Consider a Repeat Bacterial Culture

A bacterial culture involves a clinician taking a swab of the pus or crusts on your scalp to see what type of bacteria is growing and which anti-bacterial medications might work best against it.

• Treatment Failure: A culture may be considered if your condition does not improve with standard antibiotics targeting Staphylococcus aureus, the most common bacteria involved in FD ^[2]. For example, one clinical study found that some patients who do not respond to standard treatments may have a gram-negative bacterial infection ^[18]. This finding may lead your clinician to consider a different type of antibiotic if the culture and susceptibility results support it.
• Severe Relapses: If you experience a severe flare, a new culture may help identify currently recoverable organisms and guide susceptibility-based treatment ^[7]. Keep in mind that a positive culture just shows what is growing on the surface—which could be normal bacteria living on the skin (colonization)—and does not automatically prove bacteria are the sole cause of the flare.

How to get the most accurate culture results

For the best results, a culture should be taken by a clinician from a fresh, draining pustule or a hemorrhagic crust (a crust with blood and pus) ^[7][5]. Ideally, this sample is taken before you start a new course of antibiotics, because medications can hide the presence of bacteria and result in a false-negative (showing no bacteria when there actually is) ^[19].

Important Safety Note: Do not stop, delay, or interrupt your currently prescribed treatments without your doctor’s explicit instructions in order to get a culture.

Comparing Biopsy and Culture